The operating principle of the cranes with movable arms, namely which are suitable to accomplish linear or rotational movements or of the composite type is known. In case of the stationary installations, the lifting and the furling operations of the hoist block occurs without any movement of the structure which houses the hanging pulleys of the hoist block, therefore it is not possible to use the movement of the arm in order to retaining in a safe condition the block, both during the movements of the load and, successively, when the load reaches its rest position.
In particular the cranes for life boats are characterized by the fact that the boat must always remain in an hooked condition to the blocks of the crane (one or two blocks depending on the fact that the boat is provided with one or two hooks). Therefore the need is present to be able to loosen the lifting cables in order to avoid the permanent deformation and/or the damage of the same if permanently kept under a tensioned condition.
Moreover the current rules for the safety of life at sea (SOLAS) prescribe that the life boats may be put into the sea in any longitudinal arrangement condition (pitch angle) and transverse arrangement condition (roll angle) of the ship, within the maximum provided limits, also in a combined condition one another.
Therefore it is necessary for the block hooking device to ensure a strong and reliable retaining action of the block in any arrangement condition which is provided by the rules and to allow the opening the device and the release of the block also under load conditions. The safety prescriptions impose that the unlock of the block must occur manually or due to the force of gravity and without any contribution of other energy sources which depend on the ship.
Solutions are known, always in conformity with the present law, in which the unlock of the block occurs by means of automatic systems which are supplied by means of a permanently stored energy reserve, as gas pressurized hydraulic devices.
The reliability of the device and therefore of crane itself also depend on the possibility that the hooking of the block occurs in an automatic way, namely without a specific intervention of the operator, any time the block, and therefore the boat, reaches the provided rest position.
In order to avoid accidental lowering of the boat which is arranged in its rest position on the ship, a safety device function against accidental lowering must be provided too.